COASTAL SEASONALITY DURING CLOSURE OF THE ISTHMUS OF PANAMA

Closure of the Isthmus of Panama has been a central focus for studies into the response of biota to oceanographic change. However, the biological data is currently much better studied, both spatially and temporally, than the environmental data. Oceanographic studies are usually either broad in time or in space. In this study we use a new technique to investigate levels of seasonality associated with the closure of the isthmus using cupuladriid bryozoans. Cupuladriids are extremely abundant throughout the Neogene of Central America and studies on Recent colonies show that their skeletons record levels of ambient seasonality discernible through morphometric analysis. We used the extensive fossil cupuladriid collections of the Panama Paleontology Project (PPP) to investigate patterns of seasonality in the Caribbean and eastern Pacific over the last 10 million years. Data were restricted to either one or two PPP samples, thereby allowing regional-scale environmental inferences to be made. The results reveal the general pattern of diverging environments during isthmus closure that have led to Pacific waters being highly seasonal and instable due to upwelling and Caribbean waters being aseasonal and highly stable with no upwelling. These changes were punctuated by considerable regional heterogeneity and a substantial drop in Caribbean seasonality during the last 2Ma. Indeed, present-day Caribbean stability was not achieved until the end of the Pliocene, suggesting that some Caribbean coastal waters were still being affected by upwelling after final closure.